Rust 2024 let-chains to simplify wait Conditions

[clux]

Bumps MSRV to 1.88.0 and converts existing let-chainable code to use let-chains. Rust 2024 support was added in separately in #1856, and we can use it properly now since 1.88 is like 3 versions behind.

Was originally playing around with it, but actually found that it solves most of the the ergonomic problem with the wait API that I tried to solve earlier in #1498 - it's not quite as elegant but it also doesn't come with any of the problems and is non-breaking.

---

But if the general setup (outlined in the first commit) is acceptable, I'll rebase at 1.90.

Waiting for #1856 to merge. Now ready.

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.7%. Comparing base (8b31a53) to head (b33a2c4).

Files with missing lines	Patch %	Lines
kube-client/src/config/file_config.rs	38.5%	8 Missing ⚠️
kube-derive/src/custom_resource.rs	46.7%	8 Missing ⚠️
kube-client/src/client/auth/mod.rs	16.7%	5 Missing ⚠️
kube-runtime/src/utils/backoff_reset_timer.rs	57.2%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1792     +/-   ##
=======================================
- Coverage   74.7%   74.7%   -0.0%     
=======================================
  Files         84      84             
  Lines       7951    7950      -1     
=======================================
- Hits        5939    5938      -1     
  Misses      2012    2012             

Files with missing lines	Coverage Δ
kube-client/src/config/file_loader.rs	77.5% <100.0%> (ø)
kube-core/src/discovery.rs	95.6% <100.0%> (ø)
kube-core/src/params.rs	85.4% <100.0%> (ø)
kube-core/src/schema.rs	95.8% <100.0%> (ø)
kube-runtime/src/wait.rs	94.8% <100.0%> (ø)
kube-runtime/src/watcher.rs	43.8% <100.0%> (ø)
kube/src/lib.rs	60.6% <100.0%> (ø)
kube-runtime/src/utils/backoff_reset_timer.rs	72.5% <57.2%> (ø)
kube-client/src/client/auth/mod.rs	50.0% <16.7%> (ø)
kube-client/src/config/file_config.rs	77.6% <38.5%> (ø)
... and 1 more

... and 4 files with indirect coverage changes

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